Polyolefin resin, for example polypropylene, is one kind of thermoplastics widely used due to desirable processing property, chemical resistance, electrical properties and mechanical properties. Thermoplastic refers to a plastic which can be softened to flow upon exposure to sufficient heat but will retain its solidified state upon cooling. Thermoplastic can be used to manufacture various kinds of injection-molding items, for example, daily necessities, medical devices, electrical appliances and plastic pipes for use in vehicles and tubes.
During solidification and crystallization of a polymer melt, a long relaxation time is needed for long-chain of macromolecule to change from a disorder state to an ordered crystalline state. Crystallization rate and crystallization temperature is generally low, which may lead to formation of big-size spherocrystals and obvious interfaces between different spherocrystals. The obvious interfaces between different big-size spherocrystals may induce different internal stress. Upon impact, cracks may form at the interfaces, which may further lead to break of the solidified polymer. When nucleating agent is added into the polypropylene melt, the nucleating agent can provides adequate crystal nucleus. Therefore, during cooling, the polypropylene melt can form much more small-size, orderly arranged, compactly and evenly dispersed spherocrystals. Accordingly, the internal stress generated is small and disperse, which can improve mechanical properties and thermal properties of polypropylene and further promote the application of polypropylene.
Generally, rigidity of polypropylene is represented by bending modulus, and toughness of polypropylene is represented by impact strength.
To modify a conventional polypropylene as a functional material having desirable physical properties, for example, high rigidity, desirable impact resistance and thermal stability, nucleation modification is one of the most simple and effective methods. Adding nucleating agent(s) in polypropylene can induce polypropylene to form orderly arranged and well-dispersed small spherocrystals, which may improve the physical properties of polypropylene. Different nucleating agent may induce polypropylene to form different crystalline state, and further present different physical properties. However, it may be seen that each nucleating agent having specific molecular structure has its advantages and disadvantages, i.e. each nucleating agent can hardly simultaneously improve various kinds of physical properties of polypropylene. For instance, phosphate nucleating agents (e.g. sodium 2,2′-methylene-bis (4,6-di-tert-butylphenyl) phosphate and aluminum bis[2,2′-methylene-bis (4,6-di-tert-butylphenyl)] phosphate) and carboxylate nucleating agents (e.g. aluminum p-tert-butyl benzoate hydroxide) can improve the rigidity and thermal deformation temperature of polypropylene. However, these nucleating agents can hardly improve the toughness of polypropylene.
Different nucleating agents may differently improve mechanical properties and/or thermal properties of homo polypropylene 1120 available from FORMOSA PLASTICS CORPORATION, TW. Almost all of the nucleating agents can improve the bending modulus and thermal deformation temperature of homo polypropylene 1120, but cannot significantly improve the impact strength of homo polypropylene 1120 at ambient temperature.
Dibenzylidene sorbitol nucleating agent (DBS) can reduce haze of polypropylene. However, dibenzylidene sorbitol nucleating agent cannot contribute to improvement of other properties of polypropylene. It is difficult to use a single nucleating agent to improve the impact strength and bending modulus of polypropylene. The nucleating agent composition according to the embodiments of the present invention is aimed to increasing the toughness and rigidity of polypropylene at the same time.
CN 101358012 discloses a composition containing 0.2 to 0.3% by weight 1,3,2,4-dibenzylidene sorbitol and remaining residual random copolymerized polypropylene. The composition of CN 101358012 can be used to improve the rigidity and thermal equilibrium temperature of polypropylene, which is different from the nucleating agent composition according to the present invention.
CN 101190983A discloses a method of producing high-gloss heat-resistance polypropylene having an improved thermal deformation temperature. However, bending modulus of polypropylene is not desirable. Polypropylene modified by the nucleating agent composition according to the embodiments of the present invention not only has improved thermal deformation temperature, but also has improved bending modulus and impact strength.
What is needed, therefore, is to provide a polypropylene nucleating agent composition which not only can remarkably improve the bending modulus and thermal deformation temperature of polypropylene, but also can remarkably improve the impact strength of polypropylene.